Double patterning is a technology developed for lithography to enhance feature density in integrated circuits. Typically, for forming features of integrated circuits on wafers, lithography technology is used, which involves applying a photo resist, and defining patterns in the photo resist. The patterns in the photo resist are first defined in a lithography mask, and are implemented either by the transparent portions or by the opaque portions in the lithography mask. The patterns in the patterned photo resist are then transferred to the manufactured features, which are formed on a wafer.
With the increasing down-scaling of integrated circuits, the optical proximity effect posts an increasingly greater problem. When two separate features are too close to each other, the optical proximity effect may cause the features to short to each other. To solve such a problem, double patterning technology is introduced. The closely located features are separated to two masks of a same double-patterning mask set, with both masks used to form features that would have been formed using a single mask. In each of the masks, the distances between the features are increased over the distances between the features in the otherwise a single mask, and hence the optical proximity effect is reduced, or substantially eliminated.
Conventional double patterning technologies require two photo lithography processes. In some conventional double patterning processes, the patterns in a first lithography mask of a double-patterning mask set is transferred to a sacrificial layer, which may be a hard mask layer, using a first photo lithography process. The patterns in a second lithography mask of the same double-patterning mask set are then transferred to the sacrificial layer using a second photo lithography process. The patterns in the sacrificial layer are then used to form the desirable features.
In other conventional double patterning processes, the patterns in a first lithography mask of a double-patterning mask set is transferred to a first photo resist. The patterned first photo resist is then protected (referred to as litho-freeze). The patterns in a second lithography mask of the same double-patterning mask set is then transferred to a second photo resist at a same level as the first photo resist. The patterns of the first and the second photo resists are then transferred to form desirable features.
In yet other conventional double patterning processes, sacrificial patterns are first formed, and then spacers are formed on the sidewalls of the sacrificial patterns. The sacrificial patterns are then removed, leaving the spacers. As a result, the pattern density of the spacers is doubled over that of the sacrificial patterns, and the pitch of the spacers is reduced to a half of that of the sacrificial patterns. A separate lithography process is then performed to pattern the spacers. The patterns of the spacers may then be transferred to form desirable features.
In the conventional double patterning processes, two lithography steps are needed. Due to the mask misalignment between the two lithography steps, the relative positions of features formed using the first and the second lithography masks may deviate relative to each other. This will impact the RC and the timing behavior of the resulting circuits.